tidy: Add a rule ensuring that // comments are followed by a space in Rust (#38698)

This shows up sometimes in code reviews, so it makes sense that tidy
enforces it. `rustfmt` supports this via comment normalization, but it
does many other things and is still an unstable feature (with bugs).

Testing: There are new tidy tests for this change.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/element.rs

@@ -2940,8 +2940,8 @@ impl Element {
                 HTMLElementTypeId::HTMLAnchorElement,
             )) => element.has_attribute(&local_name!("href")),
 
-            //TODO focusable if editing host
-            //TODO focusable if "sorting interface th elements"
+            // TODO focusable if editing host
+            // TODO focusable if "sorting interface th elements"
             _ => {
                 // Draggable elements are focusable.
                 element.get_string_attribute(&local_name!("draggable")) == "true"